It was shortly after 8.15pm on Sunday that David Bunting first saw the two girls walking along the edge of the road near the village of Bishopton in Renfrewshire, Scotland. Bunting, 23, a landscape gardener from nearby Erskine, spotted the pair again 10 minutes later as he made his way home. By this time, they were near the junction with a local hospital, which lies in the shadow of the Erskine bridge.

"They were just walking along talking, a normal couple of girls," he said. "They looked quite happy."

Bunting thought he recognised the girls as the same two he had seen the previous week in Bishopton. He had guessed then that they had come from the nearby Good Shepherd care unit because it was not unusual, he said, to see girls who had apparently run away from the centre. On that occasion, the pair ran off when a car approached.

When the news broke that two girls had jumped to their deaths from the bridge, Bunting found their photographs online.

"I've been asking myself, there must have been someone among the cars on that bridge to see two girls go towards the railings. I would like to think if I'd been on the bridge just a bit later I could have prevented it somehow."

There has been much soul-searching since Niamh Lafferty, 15, and Georgia Rowe, 14, slipped away from the Good Shepherd care unit and jumped together from the 50-metre-high bridge over the river Clyde. Those who work with vulnerable children across the UK say the tragedy has shaken the sector to its core.

Ian Milligan, assistant director of the Scottish Institute for Residential Childcare, said the incident had "caused such shock we are still thinking about what we are going to do".

He said the tragedy would remind workers to recognise the degree of emotional pain that young people in care are in.

"We have to try to understand the depth of emotional trauma they have experienced. We already do that, but this incident has heightened the need to be aware of their backgrounds and the troubles that young people in care carry around with them."

Figures suggest that 45% of children in care are assessed as having mental health problems, compared with 10% of the general population. The proportion is higher for children living in residential care. Suicides within the care system are relatively rare, but not unheard of. A Scottish government study on suicide found that of the 50 looked-after children who died between 1997 and the end of 2001, 11 had killed themselves.

Both Niamh and Georgia had chaotic lives. Niamh, from Helensburgh, had been struggling with the death of her 16-year-old boyfriend from a reported drug overdose, and Georgia, originally from Hull, was said to have faced a "range of difficulties in her life".

The Care Commission has announced an official inquiry, which will take between six and eight weeks. It is likely that a key focus will be to discover how Niamh and Georgia were able to leave the Good Shepherd Centre unseen.

The facility, funded by the Cora Foundation, a Roman Catholic charity, offers residential and daycare services for local councils, social services departments and children's courts across the UK. Following a cash crisis prompted by a drop in referrals earlier this year, the residential unit faced closure, but was eventually reduced in size. Both Niamh and Georgia were resident in the centre's open unit, but would have required authorisation to leave.

David Bunting, who has lived in the area for five years, said it was common to see girls from the centre in Bishopton. "Bishopton is a very small place; you know everybody's face. Good Shepherd girls stand out. They normally get as far as the Co-op before they're caught. If you go through Bishopton on a Friday night you see them at the shop trying to get cigarettes and drink. It's quite common."

A spokesman for the centre said girls needed permission to leave but it was an open unit and they were not kept "under lock and key". Security guards would not be allowed to physically stop them. He could not comment on whether Niamh and Georgia had left the unit before Sunday.

The Good Shepherd facility was investigated by both police and the Care Commission last year after some residents alleged they had been abused and beaten in a "punishment room". The allegations were rejected and no further action was taken. The same year, a separate complaint was made that included allegations over staff shortages and the suitability of some staff. That complaint was partially upheld and recommendations were made regarding staff practice, training and recording of restraint procedures.

A spokesman for the centre said all these recommendations had been addressed, and that the Care Commission's most recent report had praised the centre's approach and management as good and very good in all areas. The family of one of the girls, he said, had nothing but praise for staff at the centre.

The spokesman said an internal inquiry was ongoing, covering "every possible detail of everything that happened that fateful evening". Staff and residents at the centre are said to be traumatised by the event and are receiving pastoral care.